This invention is directed to a cushioned mast for lift trucks which acts to damp out the loping or pitching characteristics encountered when the truck negotiates a bump or other road discontinuity. More particularly, the invention is directed to a shock absorbing means in the form of an accumulator hydraulically connected to the mast tilt cylinder to permit the load and mast function as a vibration absorber.
Currently, many pneumatic tire and fork lift trucks exhibit a loping or pitching characteristic when traveling over bumps and irregular or rough terrain while carrying a load on forks extending ahead of the vehicle. Typically, a pitching or loping action occurs about the front tires when the truck negotiates a bump or other road discontinuity. The resiliency of the pneumatic tire somewhat absorbs the shock of the bump but also causes the vehicle to pitch or lope about the tires. This pitching or loping may be further aggravated as the rear tire or tires then encounters the same bump by moving in a forward direction of travel.
This pitching or loping action causes several problems. First of all, the action commonly results in insufficient loading on the rear tires which are usually steerable and thereby prevents effective control. As the speed of the vehicle is increased, the tendency to lift off at the rear increases. This severely limits travel speed and thus, vehicle productivity.
Secondly, operator fatigue and discomfort is produced by the severe pitching and loping action. Thirdly, the dynamic action of the load positioned on the lift forks extending from the front of the vehicle and the mass of the vehicle extending rearwardly increases the stress loading imparted on the vehicle components. This increases the chance of vehicle breakdown and consequent lost time for repairs.
Prior attempts to solve the thus-described problem have taken the form of the application of systems similar to that disclosed in U.S. Pat. Nos. 2,672,995 and 3,122,246 in lift trucks. These systems, which basically include installing an accumulator in the lift circuit have not been entirely successful. This is most likely due to the basic geometry of the fork lift truck and mass which differs from that of the bucket and arm arrangements of the aforementioned patent devices. The basic geometry of the fork lift truck produces a nearly pure pitch about the axle of the front wheel which differs from that encountered with the subject devices.